(This is partly trolling but partly entirely true - the EU is aware of the difficulties of interstate business and factors in reducing them, while the US has lots of market-fragmenting rules, especially in banking and healthcare)
Each state requires different health insurance plans, so the employer would need to supply 50 different health insurance plans.
It would roll back local minimum wage hikes.
Forming unions would require national coordination inside of local.
Many states have extra protections for the workers, like California protecting IP generated outside of office workers.
National politics are very unstable and the rules and regulations for everyone would be under threat of change after every election that changes the balance of the president, the house and the senate.